1. Technical Field
This invention relates to a method for preparing a partially tert-butoxylated poly(p-hydroxystyrene) which is useful as resist materials and various other functional materials.
2. Prior Art
As base polymers for chemically amplified resist materials, Japanese Patent Application Kokai (JP-A) Nos. 45439/1984 and 115440/1987 disclose resins wherein some of the hydroxyl groups of polyhydroxystyrene are replaced by tert-butoxycarbonyloxy groups (abbreviated as t-Boc groups). These resins are thermally unstable, complicated to synthesize, and expensive because an expensive reagent must be used for introducing t-Boc groups.
One solution to this problem is partially tert-butoxylated poly(p-hydroxystyrene) which is a polyhydroxystyrene having some of its hydroxyl groups replaced by tert-butoxy groups (abbreviated as t-BuO groups). According to JP-A 211258/1992, it can be prepared by the following three methods.
A first method is by copolymerizing tert-butoxystyrene with hydroxystyrene to form partially tert-butoxylated poly(p-hydroxystyrene). Since hydroxystyrene is a monomer having high spontaneous polymerization ability, monomer handling and polymerization control are difficult.
A second method is by reacting polyhydroxystyrene with isobutene to introduce t-BuO groups to form partially tert-butoxylated poly(p-hydroxystyrene). The method follows a complex synthesis route involving polymerization of a hydroxystyrene derivative having a protected hydroxyl group, removal of the protective group from the resulting polymer, and introduction of t-BuO groups by addition reaction with isobutene. Since addition of isobutene does not occur in proportion to its charge, it is difficult to introduce a desired amount of t-BuO groups.
A third method is by effecting partial elimination reaction of poly(tert-butoxystyrene) to form partially tert-butoxylated poly(p-hydroxystyrene). It is simply required that some t-BuO groups be eliminated from poly(tert-butoxystyrene), which is synthesized by radical or anionic polymerization of tert-butoxystyrene, in the presence of an acid catalyst. However, since the reaction proceeds in a catalyzed manner, partially tert-butoxylated poly(p-hydroxystyrene) having an amount of t-BuO groups as designed cannot be prepared simply by controlling the amount of the acid catalyst added for reaction. Since a t-BuO group content is one of the important factors governing resist properties such as a dissolving rate in a developer, optimization of a t-BuO group content is very important for the manufacture of high sensitivity, high resolution resists. It is thus necessary to control the t-BuO group content exactly to the design value.
An attempt was made to perform quantitative analysis of a degree of elimination of t-BuO groups during reaction and terminate the reaction when a desired degree of elimination is reached. For the analysis of a degree of elimination, proton-NMR, infrared (IR) absorption spectroscopy and gas chromatography are employed. Although the t-BuO group content of the resulting partially tert-butoxylated poly(p-hydroxystyrene) can be directly determined by proton-NMR and IR spectroscopy, the polymer must be thoroughly purified in order to enable this analysis, taking a substantial time for measurement. This is inadequate for quick observation of a degree of progress of elimination reaction. In the case of gas chromatography, the concentrations of isobutene, tert-butyl alcohol and tert-butyl chloride resulting from elimination reaction are measured, and a t-BuO group content of the resultant partially tert-butoxylated poly(p-hydroxystyrene) is determined from these concentrations. This method allows for quick analysis because of direct measurement of the reaction solution. However, precise measurement with a sufficient degree of reproducibility is difficult because the t-BuO group content of partially tert-butoxylated poly(p-hydroxystyrene) is calculated indirectly from the amounts of the eliminated compounds whose concentration in the reaction solution is very low.